In a known inverter circuit of this kind (DE-AS 16 13 776) the voltage regulator is influenced by a control quantity created by the comparison of a desired value which is predetermined as a voltage signal with a voltage value which is proportional to the existing value of the input voltage of the inverter. The existing value of the input voltage is also fed to the frequency emitter which supplies the inverter with frequency signals proportional to the voltage. In similar circuits, a departure can be made from the rigid proportionality to take into account the slip compensation, a current-dependent compensation or the like. By reason of this proportionality, the asynchronous motor can be operated at a maximum torque over the entire speed range up to the nominal frequency. There is the additional advantage that, upon a fall in the desired value for reducing the motor speed, the frequency of the motor that is now operating as a generator does not follow the desired value setting but remains linked to the existing voltage value. Consequently, as referred to the frequency of the A.C. voltage fed to the generator, the latter operates at a speed which is only slightly above the synchronous speed. As a result, there are only very small currents which cannot influence the inverter circuit. However, this manner of operation is limited to the range up to nominal frequency because the upper limit of the supplied voltage is generally equal to the nominal voltage.
Inverter circuits are also known (DE-OS 26 48 150), in which operation is possible above the nominal frequency, the motor being operated at maximum power in the range between the single and double nominal frequency. For this purpose, however, a comparatively expensive computer circuit is necessary for maintaining a constant slip frequency at every operating point.